Fix mojo Shared Buffer documentation example

Per the current code, to create a mojo::ScopedSharedBufferHandle, one
should call the mojo::SharedBufferHandle::Create.  Update README docs
code example to say that.

Bug: 775809
Change-Id: Icfc86485923d1ec5fb7bb9d1b7b7125928af9e5f
Reviewed-on: https://chromium-review.googlesource.com/781310Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518401}

mojo/public/cpp/system/README.md

@@ -127,7 +127,7 @@ A new shared buffers can be allocated like so:
 
 ``` cpp
 mojo::ScopedSharedBufferHandle buffer =
-    mojo::ScopedSharedBufferHandle::Create(4096);
+    mojo::SharedBufferHandle::Create(4096);
 ```
 
 This new handle can be cloned arbitrarily many times by using the underlying